A R T H R O – “ J O I N T E D ” P O D A “ F E E T ”

Arthropoda

The arthropods are a group of animals which has attained the greatest biological success – largest number of species and individuals and occupy the greatest number of habitats. At least 75% of animals are arthropods!

All arthropods posses three key features:

A tough chitinous exoskeleton.

A series of jointed appendages.

A segmented body.

Arthropoda is divided into four subphyla:

Trilobita: an extinct group of marine arthropods that flourished in ancient seas.

Chelicerata: includes spiders, ticks, mites, scorpions, and horseshoe crabs. Name of subphylum is derived from a characteristic pair of appendages –the cheliceras

Crustacea – most are aquatic arthropods with gills, mandibles, 2 pairs of antennae, and 2 compound eyes. Eg. Crabs, lobsters, shrimp, barnacles, pill bugs.

Uniramia: Most arthropods are in this group –centipedes, millipedes, and all insects. Characterized by unbranched (uniramous) appendages

Habitat:

Found everywhere: all parts of the sea, freshwater, land and air

Structure:

Very diverse, but all have a tough exoskeleton made of chitin, jointed, appendages, and a segmented body. Usually the body has a recognizable head, thorax, and abdomen, but the first two of these may be fused into a cephalothorax. Triploblast, coelomateanimals with an anus.

Ingesting Food:

Every mode of feeding is seen – herbivores, carnivores, parasites, filter feeders and detritus feeders. The appendages of arthropods have evolved in ways that enable these diverse methods of feeding.

Elimination:

Undigested food leaves through the anus.

Internal Transport:

A well-developed heart pumps blood through an open circulatory system.

Tracheal tubes in most terrestrial arthropods carry respiratory gasses by diffusion to all tissues in the body.

Respiration:

Three basic types of respiratory structures are found – gills in aquatic arthropods such as crustaceans, book lungs are found in spiders and their relatives, and tracheal tubes are found in most terrestrial arthropods.

Excretion:

Most terrestrial arthropods dispose of nitrogen-containing wastes by using a set of Malphigiantubules that remove wastes from the blood and then add them to undigested food before it leaves via the anus. Small excretory glands may be found at the base of the legs in some terrestrial species.

Excretion Con’t:

In aquatic arthropods, cellular wastes diffuse from the body into the surrounding water via the gills. Many aquatic arthropods also eliminate nitrogen –containing wastes through a pair of green glands that empties through a pair of openings in the head.

Locomotion:

Arthropods have well-developed muscle systems. The muscles are attached to the exoskeleton and their contraction results in walking, flying and swimming motions.

Sensing Environment:

Arthropods have simple sense organs such as statocysts and chemical receptors, but most also have sophisticated sense organs such as compound eyes, and well-developed senses of taste, touch, and hearing. Eg. Flies can taste with their legs and antennae, and their compound eyes have over 2,000 individual lenses.

Coordination:

Most arthropods have well-developed nervous systems. All have a brain that consists of a pair of ganglia in the head, and a ventral nerve cord with a ganglion for each original body segment.

Reproduction:

Reproduction in most arthropods is simple. Sexes are separate and fertilization takes place inside the body of the female (internal fertilization).

Development may involve either complete or incomplete metamorphosis.

Growth and Development:

In order to grow all arthropods must molt or shed their exoskeletons.

When it is time to molt the epidermis of the arthropod digests the inner part of the exoskeleton and absorbs much of the chitin in order to recycle the chemicals in it.

Growth and Development Con’t

Then the arthropod secretes a new exoskeleton beneath the old one. This new exoskeleton is soft and pliable; also it is larger than the animal to provide room for growth.

The arthropod casts off its old exoskeleton by expanding its body and pulling itself completely out of the exoskeleton. The arthropod must wait for the new exoskeleton to harden – this can take from a few hours to a few days. During this time the animal is vulnerable to attack and must hid from predators.

Metamorphosis:

Animals of some species experience spectacular but predictable changes in body form and activity at some point during their post-embryonic development. This phenomenon is metamorphosis.

Any aquatic tadpole changing to a terrestrial frog or a crawling caterpillar that becomes a flying butterfly are common example of metamorphosis.

Aside from a very few that have no significant transformations between the embryo and the adult, insects species follow one of two patterns of post-embryonic development.

Complete metamorphosis:

Involves a larval and pupal stage between the egg and the adult. All four stages are very different from each other. Eg. Bees, beetles, butterfly.

Egg: Embryo developing in a protective case.

Larva: Often worm – like and equipped to eat. Often inhabits an entirely different habitat and eats an entirely different diet from the adult. Eg. Aquatic mosquito larvae feeds off of algae while adult flies and sucks blood (female) or plant juice (male). This serves to reduce competition for resources between juveniles and adults of the same species.

Pupa: Non locomotive and non feeding stage. Materials within the pupal case (larval body) are recycled to create adult body parts. When animals break out of its pupal exoskeleton it is essentially a functioning adult.

Adult: Sexually active reproducing stage of the life cycle. May have a very short life and be nonfeeding.

Incomplete metamorphosis:

Involves fewer distinct life stages. The animal that hatches from the egg case is a nearly perfect miniature of the mature adult called nymph. Between the first nymph stage and the adult, the animal may pass through several molts as it grows and takes on adult feature and function such as reproductive organs and larger wings. Because distinct changes occur, this is a truly metamorphic event. Eg. Grasshopper, Dragonfly